Moisture curable compositions crosslink upon exposure to moisture, typically atmospheric, to give cured materials that find applications as adhesives, sealants, potting materials for electronics, optical materials, components of light-emitting devices, silicone foams, weatherstrip coatings, and paper release coatings. These compositions comprise polyorganosiloxanes and non-silicone polymers having moisture-reactive groups, such as alkoxysilyl groups. The curing of such moisture-curable compositions is catalyzed by metal complexes, including compounds employing metals such as Sn, Ti, or Zn, and non-metal-based catalysts, such as amines, amidines, and guanidines. Organotin compounds such as for example dibutyltin dilaurate (DBTDL) are widely used as condensation cure catalysts to accelerate the moisture-assisted curing of a number of different polyorganosiloxanes and non-silicone polymers having reactive silyl groups.
The cure chemistry of these moisture-curable compositions can vary based upon the nature of the polymers and their moisture-curable groups. For example, alkoxysilyl groups first hydrolyze to give silanol functionalities, which then condense with the extrusion of water to give the siloxane network. Such compositions typically comprise an alkoxysilyl- or silanol-functional polymer and a crosslinking agent. Tri- and tetraalkoxysilanes are commonly used as crosslinking agents and will react with water or directly with silanol groups to crosslink the system. However, for compositions comprising hydridosilyl groups or both hydridosilyl and silanol functionalities, such a crosslinking agent is not required. In fact, due to the multitude of hydridosilyl groups present, the hydridosilyl-containing compound is often referred to as the crosslinking agent. In these compositions, hydridosilyl groups may react with water to give silanol functionalities or they may react directly with silanol groups to form siloxane bonds with extrusion of hydrogen gas. For transition-metal-catalyzed compositions comprising a hydridosilyl-containing compound, inhibitors are commonly used to ensure adequate shelf life or pot life.
Catalyst selection varies somewhat for these two classes of moisture-curable compositions, however, tin-based complexes, particularly DBTDL, efficiently catalyze moisture cure for both classes. Environmental regulatory agencies and directives, however, have increased or are expected to increase restrictions on the use of organotin compounds in formulated products. For example, while formulations with greater than 0.5 wt. % dibutyltin presently require labeling as toxic with reproductive 1B classification, dibutyltin-containing formulations are proposed to be completely phased out in consumer applications during the next three to five years.
The use of alternative organotin compounds such as dioctyltin compounds and dimethyltin compounds can only be considered as a short-term remedial plan, as these organotin compounds may also be regulated in the future. It would be beneficial to identify non-tin-based accelerators that accelerate the condensation curing of moisture-curable silicones and non-silicones.
Substitutes for organotin catalysts should exhibit properties similar to organotin compounds in terms of curing, storage, and appearance. Non-tin accelerators would also desirably initiate the condensation reaction of the selected polymers and complete this reaction upon the surface and may be in the bulk in a desired time schedule. There are therefore many proposals for the replacement of organometallic tin compounds with other metal- and non-metal-based compounds. These new accelerators have specific advantages and disadvantages in view of replacing tin compounds perfectly. Therefore, there is still a need to address the weaknesses of possible non-tin compounds as suitable accelerators for condensation cure reactions. The physical properties of uncured and cured compositions also warrant examination, in particular to maintain the ability to adhere onto the surface of several substrates.